Automatic gain control



y 1949- F. P. MASON 2,476,990

AUTOMATIC GAIN CONTROL Filed Dec. 15, 1944 MAIN. OUTPU T E\ L A MPL/F/ER.

I nvenlor Fkzyimr-(Psncwm. mason A ltbrnzy Patented July 26, 1 949 s PATENT OFFICE AUTOMATIC GAIN CONTROL Frederick Percival Mason, Croydon, England, assignor to Creed and Company Limited, Croydon, England, a British company Application December 15, 1944, Serial No. 568,379

- In Great-Britain January 5, 1944:

- 7 Claims.

. I l r The present invention relates to telegraph receiving arrangements and is an improvement in or.modification of the arrangements claimed and described in United States Patent No. 2,393,921

issued to Frederick P. Mason January 29, I946. In the specification of the parent patent, of

which the present application is a continuationin-part, identified above, it has been shown that,

in telegraph transmission channels, there isa need for an automatic gain control'which will operate correctly when signalling is effected by modulating the transmitted power, which during marking'signals is of a finite amplitude, and during spacing periods, is below a predetermined value or effective zero. An example of this method of signalling is continuous-wave operation over a radio path; another example is that of 'teleprinter signals used in voice-frequency systems. A suitable automatic gain controlme'ans is described in the parent patent referred to above, but it has been found that 1. A circuit, arranged as described in the parent applicationand utilising components of normal quality, operated correctly. The degree of control obtainable, however, is small, but it enables operation to be carried out over a very narrow frequency band channel, if used in addition to a normal type of automatic volume control.

2-. The reason for the limitation in the control is that a high value of insulation resistance is necessary, and difficulty is experienced in attaining a sufiiciently high value with components of normal" quality;

3. In order to improve the range of control, the circuit must be rearranged to' avoid the difficulties caused by the low insulation inherent in present invention is to overcome the above difficulty'. The principle utilised is based on the following conception of the fundamentals of automatic gain control in communication channel's.

It is the purpose of an automatic gain control a (A- G. C.) to eliminate the effect of variation in the gain of a communication channel. Since the channel frequency band-width demanded by signals is entirely occupied by those signals, no reference signal can be propagated which can be guaranteed to sufler the same attenuation as the signals. The sole criterion of the gain of a communication channel is, therefore, the behavior of the signais themselves.

Signalling is effected by amplitude modulation of the propagated power; Fortuitous superimposed modulations occur, caused by variations inchannel characteristics, e. g. fading in a radio channel. At the receiver it is necessary to discriminate between the intended and the fortuitous modulation. The extent of the modulation is no criterion, since either intended or fortuitous modulation may be of any depth.

It is f'oimd that in practice the rate of mod-ulation offers a satisfactory criterion as to its origin, since the rate of change of amplitude in signalling is either zero or much greater than in fading.

The particular function of an A. G. C. is, therefore, to compare the rate of change of amplitude with an arbitrary fixed rate, to compare the signal amplitude with an arbitrary fixed amplitude, and to adjust the gain of the channel so that the received signal is brought to a standard amplitude.

The necessary action under all possible. condicertain types of components. The'obi'ect of the tions is set out in Table 1'.

' Table 1 ha Bite of Action Direction of: c nga c angei 4 of ougzgit signal compared jgffggf Ad t d f h w r magni e' wi cp'eorsimp'ci o standard l Theoretically med apparatus Decreasing during 1.. Bligh Modulation of: Trans- None, since receiver gain has been-correctly set Increase receiver gaint'by a verymarkmg signal. mitter. during early portion ofsignai. few moderate steps, the total increase produced being: small, so that the receiver gain is maintained substantially constunt. 2. Low Increaseofattenuation Increase receiver-gain immediately'tu maintain Increase receiver gain by'a large oipropagatinn path. overall channelgain constant. number ofsmallzstepathetctal increase being. such that, the.

overall channel gain is mainmm substaniially constant. Zero during marking 3-. Z'ero.- N n No 5 Completion of previous action,

signal. i i b tkcmnotmthcr changei.

Table 1. --Continued D h Iclfiite of Action irection of c ange ange of outpit signal compti'ilred gg g a iffi of Ad ted f m 1 My f magnitu e wi op or s p re 7 ta Theoretically correct H I appammg Increasing during 4. High. Modulation of Trans- None,"si'nc e signal will eventually reach ampli- Decrease receiver gain by a very marking signal. mitter. tude of last signal for which receiver gain was few moderate steps, the total correctly set unless conditions 6 or 7 occur. decrease Eroduced being small, so that t e receiver gain is reestablished at the correct value, corresponding to maximum w p v V 7 signal amplitude reached. Decrease of attenua- Decrease receiver gain immediately to maintain Decrease receiver gain by a large 5. Low

tion of-propagation path.

overall channel gainconstan number of small steps, the total decrease being such that the overall channel gain is maintained substantially constant.

Marking signal much smaller in amplitude than previous signal, so that it is below the operating ampli de.

Assumed to be anab normally rapid incrcase of attenuation :during previous spacing interval.

, (a) Increase receiver gain during preceding spacmg interval, or

(0) Increase receiver gain immediately signal is received. Norr:.0wiug to absence of evidence during a.

spacing interval, action (a) may be erroneous,

but will be counteracted immediately a new marking signal reaches the operatingvalue; v .A ct10nl(b) is unattainable, since, by postula- *tl0Il, l7hB signal is inoperative. I

Marking signal much larger in amplitude normally rapid de- .Assumed to be an ab- (a) Decrease receiver-gain during preceding spac- 7 mg mterval,-or -(b)t Decrease receiver gain immediately new .3 readjusted, if necessary, im-

mediately marking signal ex ceeds normal output amplitude T Norn'. 'llie slow increase of gain 5 ensures that the receiver gain will not freeze at a valuejwhich prohibits the reception of any. future smaller slgnals.- At tall:

worst, a few marking sign will be lost immediately followthan previous sigcrease of attenuation nal. during previous e'e s marking signal esceeds standard value.

Ora-Action (ll) lS inadvisable, since, until the I '1 arrival of a marking signal, there is no evidence I as to the direction this may be such as to give rise to conditionfi, which requires the reverse action. Action (b) is practicabl of change of attenuation, and

It will'be foundthat a device arranged to carry out precisely the action shown in the-column Theoretically correct action -is impracticably, complicated; the present invention provides an approximation to the correct :conditions in a simplemanner.

In accordance with the present invention, ,a telegraph receiver having automatic gain control comprised means for varying the gain in l a manner dependent upon the rate of change of amplitude of the received signals 1 From another aspect the invention comprises a carrier wave telegraph receiver having. an automaticv gain control. arrangement.v provided withv means for adjusting the gain control volt-.1 age duringalternate halfcycles-of the carrier.v or an intermediate frequency wave. 1 In accordance with a more, specifioaspect of the invention a. telegraphlreceiver. having automatic Igain controlcomprises means. for .im I mediately reducingthe gain when the amplitude l ofthe received signals rises above a predeter mined or standard value, for vslowly increasing the gain when the amplitude of the received-sig- 55 waveform distortion. Since distortion; of the nals gradually drops, .for exa-n iple, due to fading D. C, signal, output is'to be avoided, it follows p in a radio path, betweensaid-standard .and a that the .A-. :G. C. should be confined to nonpredetermined lower value (effectively zero) and 1 rectified'signals, as any distortion of thewave for increasing the gain at a slower rate when the 5 form of the individual cycles has practically no amplitude of the received signals falls rapidly -efiect on the wave form of'the-rectifiedsignals; below the effective zero valueowing to modula- An arrangement functioning in the simplified tion of the transmitted power.

manner accordingto the invention is shown in I The action of the automatic gain control in the accompanyigdrawing. The arrangement accordance with this more specific aspect of the 7 operatesion unrectifiedsignals, and incorporates invention is shown'in the last column Adoptedl mea'ns for achieving a practically instantaneous j for simplicity of apparatus 'of'the Table 1'. decrease of gain, but increase of gain can occur It willbe observed that during a transit from I only at a maximum controlled rate. Therefore, mark to space,linstead of the gain remaining a rapid diminution of signal amplitude, due to constant, as in the theoretically correct case (see transmitter modulation, results in a relatively conditions Sand 7, Tablel) in the modifiedcas' slowin'crease of gain, and the amplitude of an the gain rises for a short time and eventually re-' output signal from the receiver can diminish to mains constant at a slightly higher value when efiective zero. quantitatively, if a signal amplithe. signal is substantially at its effective zero, tud diminishes by, say, 11 decibels per millii.1je, "at an amplitude below the normalsignal' sccor d, thecontrolled rate of lncrea'selo'f-gain-r' Thlsimperfection should have but slight eflect on the utilityofthe device- 1 The other imperfection ,in the;

increase instantaneously as would be proper under this condition, which arises if a. fade has "taken place during a long spacinglinterval, so

that a new marking signal has a lower amplitudethan that of the last marking signal, before The gain may, therefore,

the spacinginterval. ,beinadequate for the faded signal, and since the gain is allowed to increase :only at a controlledv rate, a. considerable initial portion of, the new signal may-be lost. Itappears]that'this price must be paid for the considerable simplification which is effected I by the use ofthe simplified arrangements. i

It is 'evident tl ialt :thegain may be controlled at any stage in the receiver, but itisadvantage ous' to operate on the signals before rectification, since any practical A.,G.' C. will introduce may usefully be 1 decibelper millisecond: The

ing an abnormally ra id reduction ofreceived signa strength.

simplified schemels that if a. condition is-met where a sigv nal of too low an amplitude exists but is of con stant magnitude, the gain cannotbe arranged to 7? tion, falling within the scopefof theappended; claims; will occur to those skilled in the art.

What is claimed is:

1. A telegraph receiver includi g, in combina tion, an amplifier'connected toamplify received signals,'means for controlling the amplification of the amplifier, two condensers connected between said' control means and said amplifier, means connectedto eachcondenser -for alternately charging and discharging said condensers; during successive half cycles of the signal wave, one condenser charging while the other discharges, the positive terminals of said condensers being connected to said amplification control means to vary theamplification in accordance with the average value of the charges on said condensers.

2. A telegraph receiver including, in combination, an amplifier connected to amplify received signals, means for controlling the amplification of the amplifier, a pair of condensers connected between said control" means and said amplifier, means connected to each condenser for alternately charging and discharging said condensersduring successive half cycles of the signal wave, one condenser charging while the other discharges, a pair of'equal resistances connected in series to the positive terminals. of said condensers, and means for operating said control means to vary the amplification in accordance with the voltage at the junction of said resistances. I

3. A telegraph receiver including, in combination, an amplifier connected to amplify-received signals, means for controlling the amplification of the amplifier, a pair of condensers connected to the output of the amplifier and to the input a of said control means, means connected to each condenser for alternately charging and discharging said condensers during successive half cycles of the signal Wave, one condenser charging while the other discharges, a pair of equal resistances connected in series to the positive terminals of said condensers, the junction point of said resistances being connected to said control means to vary the amplification in accordance with the potential at said junction point, andjaresistance connected between said junction pointand the negative terminals of said condensers. 1

4. A telegraph receiver including, in combina tion, an amplifier connected to amplify received signals, means for controlling the amplification of the amplifier, a pair of condensers connected to the output of said amplifier to be alternately charged thereby during successive half cycles of the signal wave, an electron discharge device con nected to each condenser to provide a discharge" path therefor, means to render each electron dis-j charge device conducting during the half cycle of the signal wave following the charging'half cycle for the condenser connected thereto, the

positive terminals of said condensers being confile of this patent:

' connected to each rectifier'to be charged thereby, duringsaid alternate halfcycles, an electron discharge device connected to each condenser to' provide a discharge path therefor, means con-r nected to the output of said amplifier for alter" nately reducing the biasing potential on each elec; tron discharge device during the half cycle of the signal wave following the charging haIf cycle for the condenser connected thereto, the positive terminals of said condensers vbeing con nected to said control means to operate, said control means 'to vary the amplification in ace, cordance with the average value of the charges,

on said. condensers.

6.- A telegraph received including, in combination, an amplifier connected to amplify receivedsignals, means for controlling the amplificationof the amplifier, means connected betweensaid amplifier and said control means for varying said control means in accordance with changes in the amplitude of received signals, the said varying means comprising a network of dual com-' ponents of two similar circuits eachhaving ca pacitance shunted by resistance and electron dis charge elements, the said two circuits being con--' nected in parallel, and means for impressing successive half cycles of the-received signal wave alternately upon each of said circuit components of. said network thereby deriving from the alter- 7. A telegraph receiverincluding, in combinav tion, an amplifier connected to amplify received signals, means for controlling the amplification; of the amplifier, means connected between said amplifier and said control means for varying said control means in accordance with changes in the amplitude of received signals comprising a network of dual components of two similar circuitseach having capacitance shunted by resistance; and electron discharge elements, thepositive sideof the capacitance elementsof said two circuits being connected to a common terminal, means for impressing successive half cycles of the re-,-. 'ceived signal wave alternately upon each of. said circuit components thereby deriving from the alternate'half cycles of the signal wave an average potential difference at said terminal, an elec,-

tron discharge device having its grid. connected to said terminal, and a resistance in the output circuit of said device across which is produced,

FREDERICK PERCIVALVMASON,

7 REFERENCES orrian 1" The following references are of UNITED sTArEsrA'r 'rs f i Date 1 Number Name V I i 1,987,470 Dijksterhuis Jan.8,'1935 2,108,899 1 Peterson Feb. 22,1938 2,143,722 Thomas Jan. 10, 1939 2,226,366 -Braden Dec. 24,1940 2,244,695 Hathaway July 10, 1941- record the Schleg'el 1 )ct. 26, 1943' 

